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  #31  
Old 03-31-2015, 07:49 PM
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Another comparison is to consider sound waves. When you add a sound wave that is an envelope of 7 volts positive peak voltage to a copy of the same sound wave that has been inverted (turned opposite and upside-down so that it at the same time is now a Minus 7 volts peak), when this mix passes through a speaker or a capacitor, the positive and negative voltages cancel. The speaker cannot move forward or backwards due to the opposing voltages.

This is also how analog Dolby Surround works. The rear channel is added to the left channel in the mix at half value. And the rear channel is added to the right channel at half value, but inverted (flipped upside-down). The Dolby Surround equation for the rear channel matrix in the resultant composite stereo signal is stated as "Left - Right" or L-R, indicating that the right channel holds the rear channel signal in an inverted
copy.

In the old days, to recover the rear channel, the decoder circuit simply inverted a copy of the right channel (to restore the inverted rear signal to it's original positive value) and added it to a copy of the left channel which contains the positive value of the rear signal. The half values of the rear channel then add to full value, while the half value of the front center signal is now inverted in the right channel by the flip in the decoder, so the front center signal then cancels out when added to the left channel containing a positive value of the front center signal.

In 1959 the broadcasters began using a process where an inverted copy of the right channel of a stereo recording is added to the original left channel to produce a Difference Signal of L-R sound information. That is done to cut down on the amounts of signal and the harmonics that are generated in radio transmission, in order to create the FM Stereo system that is compatible with mono receivers.

The original Left and Right channels are added together in-phase (without inversion of the right channel) in order to create the composite mono signal. This signal is then filtered off to remove the audio frequencies of the recordings above 15 KHz.

The Difference Signal which contains the stereo channel information is tucked in to the radio broadcast signal on a second A.M. radio carrier at 19 KHz. Which is why they filter off the high audio frequencies of a recording, to prevent interference with the Difference Signal. The center frequency of this secondary A.M. signal which contains the Difference Signal is then suppressed and is then added to the mono audio signal before being modulated to an F.M. signal. An FM radio receiver regenerates the A.M. center frequency to recover the Difference Signal.

The Difference Signal, once recovered, is added to the composite Mono Signal in phase to produce a 2X Left Channel and a cancellation of the Right Channel sound in the Mono signal. The result goes to the Left Channel amp and speaker.
The Difference Signal is inverted (flipped upside-down) and is then added to a copy of the Mono Signal to recover a 2X Right Channel and a cancellation of the Left Channel sound information in the Mono signal, and this result then goes to the Right Channel.

Both channels can also then be sent to any other circuit, including Home Theater or Surround decoders, or to the Tape Monitor for processing by equalizers and DSP (Digital Signal Processors) or for recording by other devices.

And I was asking about what happens when one subtracts the right channel from the left channel in a stereo recording shortly before that event in '59. I had been looking at my Dad's RCA Radio and T.V. repair books and the cross-sectional diagrams of waveforms with their voltage charts. And studying addition and subtraction in First Grade.
Prior to this, the only stereo broadcast experiments involved using two radios and two separate broadcasts, one for each channel.

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Last edited by sleepy; 03-31-2015 at 08:13 PM.
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  #32  
Old 03-31-2015, 08:29 PM
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yeah, my mom once told me about a course in audio science she had taken at chautauqua.

the professor set up two speakers at opposite ends of the room, turned them on individually (the sounds produced were distinct and very audible) and then together. that's when they cancelled themselves out as they met. it may be that if you sat right next to one of the speakers you would then be able to hear something... i forget what she said about that.
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Old 03-31-2015, 10:42 PM
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It's a form of acoustic cancellation. Air can behave as a capacitance, so if the signals are out-of-phase (one speaker playing an inverted waveform of the same sound), then that can happen. You will be able to hear the sound sitting next to the speakers, but the cancellation occurs at the midpoint of the room where the two waveforms cross paths. That usually requires a large spacing between the speakers so that the individual characteristics of each speaker are minimized. It is very hard to get two perfectly matched speakers. They usually have distinct anomalies.

There is also excessive speaker spacing in stereo or multichannel sound when the speakers are spaced too far apart in a large venue or room and the phantom imaging that occurs between the speakers may drop out due to acoustic cancellation, but usually due to the insufficient volume of the phantom sounds such as the "front center" sounds in a two channel stereo setup.
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Old 07-10-2016, 05:11 AM
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Old 07-10-2016, 06:43 AM
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That's a paddlin', Steve.
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Old 07-10-2016, 07:40 AM
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I'm guessing here though that would have to be fewer then 7 paddlin' per session, ahh, just thinking about it makes my buttocks clench and my bum cheeks quiver!

Kind of reminds me of the brown noise..


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  #37  
Old 07-12-2016, 05:44 AM
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Interesting , I am very eager to get the experience like this in the future
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Old 07-12-2016, 06:23 AM
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Interesting , I am very eager to get the experience like this in the future
Be careful what you are eager or wish for because it may come true...

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  #39  
Old 07-13-2016, 02:14 PM
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Old 07-13-2016, 02:20 PM
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Originally Posted by congtymayglu View Post
Interesting , I am very eager to get the experience like this in the future
R U sure about that ? Plus That is a weird look'n(&Acting) post you got there... A regular Houdini
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